We have been studying actin and myosin from amoebae of Dictyostelium discoideum in order to understand the controls on: (1) the assembly of actin- and myosin-containing structures and (2) the interaction of actin and myosin. We also wish to determine how these controls relate to external signalling which results in directed cell movement (chemotaxis). Our goals for the current year are to further examine the properties of Dictyostelium actin and myosin. Actin assembly into filaments will be examined with regard to requirements for nucleation of assembly, the role of monovalent and divalent cations in regulating assembly, and the extent to which Dictyostelium actin filaments undergo opposite-end assembly and the controls imposed upon such treadmilling. Furthermore, we will re-examine the role of cytochalasins on these various aspects of actin assembly. The Dictyostelium myosin is phosphorylated in vivo on both the heavy chain and the 18,000 MW light chain. These phosphorylations will be investigated in detail to determine their effect on the actin-activated ATPase of the myosin as well as on the assembly of the myosin into thick filaments. The possible relationship of this phosphorylation to the ability of the amoebae to chemotax toward cAMP will be explored.